This is just how ive been taught. I use the front kick mainly when in contact, like Ali has mentioned its after ive controlled (in some way) the upper body of the opponent.
Either turning them- crossing thier hands- disrupting thier balance etc
This allows you 'safe timing' to launch a straight kick at such close range.
As an example or reference - if you have Ip Mans wooden dummy book anyone can start to check out the combonations of hands and kicks in play during the form.
IME you'll find more often than not that opposite hands are used when he has a straight kick out. Of course there are exceptions to the rule so a few kicking actions within the form are using opposite hands but they are in the minority and are slightly different in application and situation based on the way i was taught.
Aside from avoiding getting crossed up with the hand and kick or just loosing force in the kick....
One can also think about structure with relation to a block and a kick.
Where is the support and how effective is it ?
If you kick with the opposite leg as say a Tan you've given to block a straight punch...your supporting leg is right under your Tan and the kick is free to issue energy supported by body movement of turning towards the punch as most do with Tan and also the knee and waist etc. The force of your turing with the Tan lends itself to the direction of force you want to give in the kick on the opposite site.
However if your kicking with the same side the Tan doesnt have as much support because the supporting leg is a further distance away from where the force is on your bridge (its not directly under)
Also you want to turn your Tan with the body but the body must remain facing foward to give the kick...the forces can contradict each other in terms of direction etc. Therefore people seem to face foward with more limited body movement whilst trying to kick at the same time... and IMO no horse no kung fu...no or little body movement = weak(er) kicks.
So the block adversely affects the kicking power IME.
It kinda makes sence...and works for me. Not all the time, nothings 100% but its worth investigating as i found it very useful.
DREW